The purpose of the proposed studies is to investigate the mechanisms regulating the biogenesis of delta-aminolevulinate synthetase (ALA synthetase) in embryonic chick liver. Initially, the precursor to the subunit of the enzyme will be identified and its subcellular site of synthesis will be determined. The processing of the precursor in vivo will be followed by pulse-labeling in order to determine the number of steps and the subcellular sites involved in its maturation. In order to localize the site(s) of action of hemin in repressing the biogenesis of ALA synthetase, the effects of this agent on the levels of messenger RNA of ALA synthetase in liver will be examined to determine whether hemin represses synthesis at the transcriptional or translational level. Possible regulation by hemin of the processing of the precursor to the subunit of ALA synthetase or of its transport into mitochondria will be investigated also. An attempt will be made to reconstitute the transport of the precursor to the subunit of ALA synthetase into mitochondria in vitro under defined conditions and to determine the factors regulating the reconstituted sequestration. Finally, the mRNA for ALA synthetase will be purified in order to synthesize a cDNA probe for the messenger to be used in future studies.